_View sometimes taken by Physiologists._--It becomes the more important for the psychologist to investigate these phenomena, inasmuch as views and theories little accordant with the true philosophy of the mind have sometimes been put forth by physiologists, in attempting to explain the phenomena in question. They have viewed the cerebral apparatus as competent of itself to produce the phenomena of thought, as _self-acting_, in the absence of the higher principle of intelligence which usually governs its operations, carrying on by a sort of automatic action, the processes usually ascribed to the mind or spiritual principle, while consciousness and volition are entirely suspended. Consciousness, in fact, is nothing but sensation, and thought a mere function of the brain. This is downright materialism, a doctrine utterly subversive of the very existence of that which we call mind or soul in man. If the cerebral organization is competent of itself during sleep to carry on those operations which in waking moments are ascribed to the spiritual element of our being, if thought is a function of the brain, as digestion is of the stomach, what need and what evidence of any thing more than merely cerebral action at any time? What, in fact, is the mind itself but cerebral activity, and what is man, with all his higher powers, but a mere animated organism? It becomes important, then, to account for the phenomena under consideration in some way more consistent with all just and true notions of the nature and philosophy of mind. _Distinction of normal and abnormal States._--Of these phenomena, while all may be regarded as intimately connected with and dependent on the state of the brain and nervous system, some seem to proceed from a normal, others from an abnormal and disordered state of the nervous and particularly the cerebral organism. Of the former class, are sleep and dreams; of the latter, somnambulism, the mesmeric state, so called, and the various forms of disordered mental action, or insanity. § I.--SLEEP. _Meaning of the Term._--What is sleep? Will the name itself afford any solution of this problem? Like most names of familiar things, we find the word descriptive of some particular circumstance or phase, some one prominent characteristic of the thing in question, rather than a _definition_--much less an explanation--of the thing itself. The word sleep, from _schlafen_, as the Latin _somnus_ from _supinus_, refers to the supine condition and appearance of the body when in this state; the relaxing of the muscles the falling back or sinking down of the frame, if unsupported. This is the first and most obvious effect to the eye of an observer, of the condition of sleep as regards the body. Further than this the word gives us no light.